In recent years, apparatuses compliant with a compression format such as MPEG (Moving Picture Experts Group), in which image information is digitally handled and is compressed using an orthogonal transform such as the discrete cosine transform and by motion compensation by utilizing a redundancy particular to image information for the purpose of achieving highly efficient transmission and accumulation of information, have been becoming prevalent for use in both distribution of information from broadcast stations and reception of information in general households.
In particular, MPEG-2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2) is defined as a general-purpose image coding format, is a standard that covers both interlaced scanned images and progressive scanned images as well as standard-definition images and high-definition images, and is currently widely used in a broad range of applications for professional use and consumer use. With the MPEG-2 compression format, a high compression rate and a favorable image quality can be realized by, for example, allocating an amount of code (bit rate) of 4 to 8 Mbps to a standard-definition interlaced scanned image having 720×480 pixels or an amount of code (bit rate) of 18 to 22 Mbps to a high-definition interlaced scanned image having 1920×1088 pixels.
MPEG-2 is mainly used for high image quality encoding suitable for broadcasting, but is not compatible with coding formats of an amount of code (bit rate) lower than that of MPEG-1, that is, a higher compression rate. With the widespread use of mobile terminals, the need for such coding formats will increase in the future, and in response the MPEG-4 coding format has been standardized. Regarding an image coding format, MPEG-4 was designated an international standard as ISO/IEC 14496-2 in December 1998.
Furthermore, standardization of the format H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)), which was initially for the purpose of image coding for video conferences, has been progressing in recent years. It is known that H.261, realizes higher encoding efficiency than previous coding formats, such as MPEG-2 and MPEG-4, though encoding and decoding according to H.26L involve a larger amount of computation. Also, as part of MPEG-4 activities, standardization for realizing higher encoding efficiency by introducing functions which are not supported by H.26L on the basis of H.261, is currently progressing as Joint Model of Enhanced-Compression Video Coding.
As the standardization schedule, a standard called H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter referred to as AVC) was designated an international standard in March 2003.
Furthermore, as an extension of the above, standardization of FRExt (Fidelity Range Extension), including encoding tools necessary for business use, such as RGB, 4:2:2, and 4:4:4, as well as 8×8 DCT (Discrete Cosine Transform) and quantization matrices defined in MPEG-2, was completed in February 2005. Accordingly, a coding format capable of favorably expressing even film noise included in movies by using AVC has been established, which is used for a wide range of applications such as Blu-Ray Discs.
However, there has recently been a growing need for encoding at a higher compression rate, for example, compression of an image having about 4000×2000 pixels, which is four times the number of pixels included in a high-definition image, or distribution of high-definition images in an environment with a limited transmission capacity, such as the Internet. Therefore, in VCEG (Video Coding Expert Group) under ITU-T, ongoing studies for enhancing encoding efficiency have been performed.
Meanwhile, for the purpose of realizing higher encoding efficiency than that of AVC, standardization of a coding format called HEVC (High Efficiency Video Coding) is currently being conducted by JCTVC (Joint. Collaboration Team-Video Coding), which is a standards group of ITU-T and ISO/IEC (see, for example, NPL 1).
In the HEVC coding format, coding units (CUs) are defined as units of processing which are similar to macroblocks used in AVC. Unlike a macroblock used in AVC, the size of a CU is not fixed to 16×16 pixels, and is specified in image compression information in each sequence.
CUs are hierarchically configured from a largest coding unit (LCU) to a smallest coding unit (SCU). That is, it may be generally considered that an LCU corresponds to a macroblock used in AVC, and a CU in a layer lower than the LCU corresponds to a sub-macroblock used in AVC.
Meanwhile, there is a coding format in which an encoding mode for encoding and outputting image data and a non-encoding mode for outputting image data without encoding the image data are provided, whether the encoding mode or the non-encoding mode is to be used is selected in units of macroblocks, and the encoding mode and the non-encoding mode can be used in combination within a single picture (see, for example, PTL 1). Also in the AVC coding format, an I_PCM mode for outputting image data without encoding the image data is supported as mb_type (see, for example, PTL 2). This is used for ensuring real-time operation of arithmetic coding processing in a case where a quantization parameter is set to be a small value, such as QP=0, and in a case where the information amount of encoded data is larger than that of an input image. Also, it is possible to realize lossless coding by using I-PCM.
Also, a method for increasing internal arithmetic has been suggested (see, for example, NPL 2). Accordingly, an internal arithmetic error caused in processing such as an orthogonal transform and motion compensation can be reduced, and encoding efficiency can be enhanced.
Furthermore, a technique in which an FIR filter is provided in a motion compensation loop has been suggested (see, for example, NPL 3). In an encoding apparatus, by obtaining an FIR filter coefficient using a Wiener filter so as to minimize an error with respect to an input image, degradation in a reference image can be minimized, and encoding efficiency of image compression information to be output can be enhanced.